/*
	226. Invert Binary Tree
	Description:
	Invert a binary tree.
	
	Example:

	Input:

		 4
	   /   \
	  2     7
	 / \   / \
	1   3 6   9

	Output:

		 4
	   /   \
	  7     2
	 / \   / \
	9   6 3   1
	
	Trivia:
	This problem was inspired by this original tweet by Max Howell:
	
	Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so f*** off.
	
*/

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    TreeNode* invertTree(TreeNode* root) {
        if ( root == NULL ) {
			return NULL;
		}
		
		TreeNode *Node = root->left;	// 交换左右节点而不是值
		root->left = root->right;
		root->right = Node;
		
		invertTree(root->left);	// 继续交换
		invertTree(root->right);
		
		return root;
    }
};